Tom Volk's Fungus of the Month for November 1997
This month's fungus is the powdery mildew, Microsphaera
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Here in Wisconsin, November means the mushroom collecting season is rapidly winding down, and we're starting to look for smaller fungi,. One that is very apparent but often ignored is a group of species collectively known as the powdery mildews. You have probably seen a powder covering some of the ornamental plants in your yard or park or woods. If you look closely, with a hand lens, you can see small black dots. These are the cleistothecia of the powdery mildew. "Cleistothecium" is the name for a closed ascocarp that contains ascospores-- the sexual (meiotically produced) spores of this fungus. However, don't let your investigation stop there. If you've got a microscope you're in for a real treat-- looking for the appendages on the cleistothecia. The highly branched appendages of Microsphaera shown here are the most spectacular.
Microsphaera penicillata is a common parasite on lilac (Syringa vulgaris). Virtually every lilac in our area is covered with conidia and cleistothecia of this fungus during the late summer to early fall, giving it a powdery appearance. It doesn't seem to be terribly harmful to the plant, but probably stunts its growth a bit by blocking sunlight to some extent, while taking some of the nutrients away from the plant. Like all good parasites, Microsphaera does not rapidly kill its host. A good (=successful) parasite, especially an obligate parasite like this fungus, keeps its host alive to produce more nutrients to feed it.
There are many other powdery mildews on many other species of vascular plants. One of my favorites is Phyllactinia (pictured here), which can commonly be found around Wisconsin on oak trees (Quercus spp.). The cleistothecia have appendages with bulbous bases. Under the right conditions of drying and wetting, the appendages raise the cleistothecium above the substrate where the spores might be more successfully dispersed. It looks to me to be similar to the spaceship on the old TV series "Lost in Space," with its leg-like appendages that lifted the spaceship above the ground. There are other species of powdery mildews that have other kinds of appendages, and even some without appendages.
The powdery mildews are obligate pathogens, requiring a living host to survive. But it doesn't usually kill the plant. The are many plant pathogens that have much more severe impacts on plants. For further information on plant pathogens that affect many types of crop plants and forests and what plant pathologists are doing about them, you should visit the home page of the American Phytopathological Society.
There are lots of interesting stories in Mycology. The fungi are interesting and do interesting and unexpected things. And people often do interesting and unexpected things with the mushrooms. So get out into the woods and forests while you have the chance. There are lots of fun things waiting for you there!
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